Long Term Evolution Systems
Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink direction and a Discrete Fourier Transform (DFT)-spread OFDM in the uplink direction. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions may be organized into radio frames of 10 ms, with each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, as illustrated in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which user equipments data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols for control purposes is illustrated in FIG. 3. The dynamic scheduling information is communicated to the user equipments via a Physical Downlink Control Channel (PDCCH) transmitted in the control region. After successful decoding of a PDCCH, the user equipment shall perform reception of the Physical Downlink Shared Channel (PDSCH) or transmission of the Physical Uplink Shared Channel (PUSCH) according to pre-determined timing specified in the LTE specifications.
LTE uses a Hybrid-Automatic Repeat Request (HARQ), where, after receiving downlink data in a subframe, the user equipment attempts to decode it and reports to the base station whether the decoding was successful, sending an Acknowledge (ACK), or not, sending a Non-Acknowledgement (NACK) via the Physical Uplink Control CHannel (PUCCH). In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data. Similarly, the base station can indicate to the UE whether the decoding of the PUSCH was successful, sending an ACK, or not, sending a NACK, via the Physical Hybrid ARQ Indicator CHannel (PHICH).
Uplink control signaling from the user equipment to the base station may comprise (1) HARQ acknowledgements for received downlink data; (2) user equipment reports related to the downlink channel conditions, used as assistance for the downlink scheduling; and/or (3) scheduling requests, indicating that a mobile user equipment needs uplink resources for uplink data transmissions.
If the mobile user equipment has not been assigned an uplink resource for data transmission, the L1/L2 control information, such as channel-status reports, HARQ acknowledgments, and scheduling requests, is transmitted in uplink resources e.g. in resource blocks, specifically assigned for uplink L1/L2 control on Release 8 (Rel-8) PUCCH. As illustrated in FIG. 4, these uplink resources are located at the edges of the total available transmission bandwidth. Each such uplink resource comprises 12 “subcarriers” (one resource block) within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources are frequency hopping, indicated by the arrow, on the slot boundary, i.e. one “resource” comprises 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa. If more resources are needed for the uplink L1/L2 control signaling, e.g. in case of very large overall transmission bandwidth supporting a large number of users, additional resources blocks can be assigned next to the previously assigned resource blocks.
Carrier Aggregation
The LTE Release 10 (Rel-10) standard has recently been standardized, supporting bandwidths larger than 20 MHz. One requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This may also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 user equipment. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable user equipments compared to many LTE legacy user equipments. Therefore, it may be useful to assure an efficient use of a wide carrier also for legacy user equipments, i.e. that it is possible to implement carriers where legacy user equipments can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 user equipment can receive multiple CCs, where the CCs have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 5.
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It should be noted that the number of CCs configured in a cell may be different from the number of CCs seen by a user equipment. A user equipment may for example support more downlink CCs than uplink CCs, even though the network is configured with the same number of uplink and downlink CCs.
During an initial access, a LTE Rel-10 user equipment behaves similarly to a LTE Rel-8 user equipment. Upon successful connection to the network a user equipment may—depending on its own capabilities and the network—be configured with additional CCs for uplink and downlink. Configuration is based on the Radio Resource Control (RRC). Due to the heavy signaling and rather slow speed of RRC signaling it is envisioned that a user equipment may be configured with multiple CCs even though not all of them are currently used. If a user equipment is configured on multiple CCs this would imply it has to monitor all downlink CCs for PDCCH and PDSCH. This implies a wider receiver bandwidth, higher sampling rates, etc., resulting in high power consumption.
To mitigate the above described problems, LTE Rel-10 supports activation of CCs on top of configuration. The user equipment monitors only configured and activated CCs for PDCCH and PDSCH. Since activation is based on Medium Access Control (MAC) control elements, which are faster than RRC signaling, activation/de-activation can follow the number of CCs that are required to fulfill the current data rate needs. Upon arrival of large data amounts multiple CCs are activated, used for data transmission, and de-activated if not needed anymore. All but one CC, the Downlink (DL) Primary CC (DL PCC), may be de-activated. Therefore, activation provides the possibility to configure multiple CC but only activate them on a need-to basis. Most of the time a user equipment would have one or very few CCs activated resulting in a lower reception bandwidth and thus battery consumption.
Scheduling of a CC may be done on the PDCCH via downlink assignments. Control information on the PDCCH may be formatted as a Downlink Control Information (DCI) message. In Rel-8 a user equipment may only operate with one downlink and one uplink CC. The association between downlink assignment, uplink grants and the corresponding downlink and uplink CCs is therefore clear. In Rel-10 two modes of CA should be distinguished. A first mode is very similar to the operation of multiple Rel-8 CC, a downlink assignment or uplink grant contained in a DCI message transmitted on a CC is either valid for the downlink CC itself or for associated (either via cell-specific or user equipment specific linking) uplink CC. A second mode of operation augments a DCI message with the Carrier Indicator Field (CIF). A DCI comprising a downlink assignment with CIF is valid for that downlink CC indicted with CIF and a DCI comprising an uplink grant with CIF is valid for the indicated uplink CC.
DCI messages for downlink assignments comprise among others resource block assignment, modulation and coding scheme related parameters, HARQ redundancy version, etc. In addition to those parameters that relate to the actual downlink transmission, most DCI formats for downlink assignments also comprise a bit field for Transmit Power Control (TPC) commands. These TPC commands are used to control the uplink power control behavior of the corresponding PUCCH that is used to transmit the HARQ feedback.
In Rel-10 LTE, the transmission of PUCCH is mapped onto one specific uplink CC, the Uplink (UL) Primary CC (UL PCC). User equipments configured with a single downlink CC (which is then the DL PCC) and uplink CC (which is then the UL PCC) are operating dynamic ACK/NACK on PUCCH according to Rel-8. The first Control Channel Element (CCE) used to transmit PDCCH for the downlink assignment determines the dynamic ACK/NACK resource on Rel-8 PUCCH. Since only one downlink CC is cell-specifically linked with the UL PCC, no PUCCH collisions can occur since all PDCCH are transmitted using different first CCE.
Upon reception of downlink assignments on a single Secondary CC (SCC) or reception of multiple DL assignments, CA PUCCH should be used. A downlink SCC assignment alone is untypical. The scheduler in the base station should strive to schedule a single downlink CC assignment on the DL PCC and try to de-activate SCCs if not needed. A possible scenario that may occur is that the base station schedules user equipment on multiple downlink CCs including the PCC. If the user equipment misses all but the DL PCC assignment it will use Rel-8 PUCCH instead of CA PUCCH. To detect this error case the base station has to monitor both the Rel-8 PUCCH and the CA PUCCH.
In Rel-10 LTE, the CA PUCCH format is based on the number of configured CCs. Configuration of CCs is based on RRC signaling. After successful reception/application of the new configuration a confirmation message is sent back making RRC signaling very safe.
Time Division Duplex
Transmission and reception from a node, e.g. user equipment in a cellular system such as LTE, can be multiplexed in the frequency domain or in the time domain (or combinations thereof). Frequency Division Duplex (FDD) as illustrated to the left in FIG. 6 implies that downlink and uplink transmissions take place in different, sufficiently separated, frequency bands. Time Division Duplex (TDD), as illustrated to the right in FIG. 6, implies that downlink and uplink transmissions take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum.
Typically, the structure of the transmitted signal in a communication system is organized in the form of a frame structure. For example, LTE uses ten equally-sized subframes of length 1 ms per radio frame as illustrated in FIG. 7.
In the case of FDD operation (upper part of FIG. 7), there are two carrier frequencies, one for uplink transmission (fUL) and one for downlink transmission (fDL). At least with respect to the user equipment in a cellular communication system, FDD can be either full duplex or half duplex. In the full duplex case, a user equipment can transmit and receive simultaneously, while in half-duplex operation, the user equipment cannot transmit and receive simultaneously (the base station is capable of simultaneous reception/transmission though, e.g. receiving from one user equipment while simultaneously transmitting to another user equipment). In LTE, a half-duplex user equipment is monitoring/receiving in the downlink except when explicitly being instructed to transmit in a certain subframe.
In the case of TDD operation (lower part of FIG. 7), there may be only a single carrier frequency and uplink and downlink transmissions are typically separated in time on a cell basis. As the same carrier frequency is used for uplink and downlink transmission, both the base station and the mobile user equipments need to switch from transmission to reception and vice versa. An aspect of any TDD system is to provide the possibility for a sufficiently large guard time where neither downlink nor uplink transmissions occur. This is required to avoid interference between uplink and downlink transmissions. For LTE, this guard time is provided by special subframes (subframe 1 and, in some cases, subframe 6), which are split into three parts: a downlink part, a Downlink Pilot Time Slot (DwPTS), a guard period (GP), and an uplink part, an Uplink Pilot Time Slot (UpPTS). The remaining subframes are either allocated to uplink or downlink transmission.